The technical properties of motor or circuit protection devices resides inter alia in detecting the temperature by means of wound bimetallic elements which are arranged in the current-conducting feed lines of electrical consumers to be monitored.
In electromechanical protection devices, in particular in circuit breakers, bimetallic or trimetallic strips are used as overload releases. In order to achieve the desired tripping characteristics, the metal strips generally either have a heating winding or a heating stack.
Heating windings are metal wires or bands which are wound around the bimetallic strip. An electrical insulator, for example glass filament fabric, is located between the bimetallic strip and the heating winding in order to prevent a short circuit of the individual heating conductor windings with respect to the bimetallic strip. The heating conductor and the bimetallic element are welded to one another at the upper end of the bimetallic strip.
Accordingly, bimetallic strips are used in electromechanical protection devices. If current is flowing through the bimetallic strips, i.e. the bimetallic strips are heated directly, they need to be connected not only mechanically but also electrically in the device. These two connections are implemented by way of welding between the bimetallic strip and the metal part. The bimetallic strip is fixed in the device by way of this metal part. At the same time, the current is passed to the bimetallic strip via the metal part.
In devices for relatively high currents, problems related to device heating and manufacturing-related and device-related obstacles has been caused by virtue of the fact that copper-plated steel materials are used for the metal part. This material can be welded easily to the bimetallic strip under certain conditions. Owing to the steel content, the material is firstly ferromagnetic and can therefore be integrated in the magnetic circuit of a short-circuit release. Secondly, the steel content also provides the required rigidity for the metal part. The thickness of the copper plating is in this case selected such that the resistance of the metal part, and therefore heating of the metal part during operation of the device, is reduced to the required extent.
It is expedient in economic and technical terms to use copper-plated steel bands for producing the metal parts only up to a certain limit. A limitation in economic terms resides in the price of the material, which is approximately twice as high as that for pure copper bands with the same dimensions. An additional cost-increasing factor in this context resides in that the waste material from stamping as mixed metal scrap only brings in a small income. There therefore results the economic necessity to minimize, in design terms, the use of this material in the device and the amount of waste produced during stamping. A technical limitation is imposed on the use of the copper-plated steel material based on the current range of the devices with an upper limit such that the thickness of the copper plating also needs to increase as the current range increases.
At the same time, the total sheet thickness of the material should remain the same so that no separate manufacturing devices including stamping and bending tools, part feeds, installation apparatuses or welding receptacles are required. This means that the steel content in the material is reduced. Above a certain limit, this results in problems when connecting the metal part and the bimetallic strip. Furthermore the metal part loses some of its required rigidity for sufficient mechanical fixing of the bimetallic strip. In the case of devices with short-circuit releases, such as circuit breakers, for example, the low steel content in the metal part results in a reduced function in the magnetic circuit of the release coil.